1. Filed of the Invention
The invention relates to a process for working up steel slags and iron carriers such as, e.g., electric furnace slags, converter slags, fine ores, dusts from steel production, mill scales for obtaining pig iron and environmentally safe slags.
2. Prior Art
From PCT/AT 96/00019 a process for producing pig iron or steel and cement clinker from slags has become known, in which iron-oxide-containing liquid slags such as, for instance, steel works slag were mixed with iron oxide carriers and lime, whereupon a ferrite slag was formed. After this, that ferrite slag was reduced in a reduction reactor under the formation of an iron bath and a sinter phase while burning carbon, whereupon the sinter phase was discharged as a clinker.
Slags have a relatively poor thermal conductivity and a thermal capacity approximately 1.5 to 2 times higher than that of iron. What is essential to the economy of a process of this kind is the heat transfer to be attained or what is called degree of afterburning. The degree of afterburning is defined as follows: ##EQU1##
By the known modes of procedure, only an insufficient degree of afterburning has been ensured so far. Nor do the known modes of procedure offer a ##EQU2##
that is sufficient for an economic mode of procedure.
Thermal efficiencies of far above 70% are attained neither in conventional blast furnace technologies nor in other processes such as, for instance, fluidized bed processes. Thus, it has already been known to blow prereduced and at least partially preheated charges into a fluidized bed along with coal, wherein coal is gasified in a fluidized bed under reduction of the charge and sponge iron is. melted and drawn off. To make things worse, such meltdown gasification reactions, as a rule, are optimized with a view to the pig iron output sought so that no environmentally safe slag is formed.
The present invention aims at providing a process of the initially defined kind, by which the thermal yield and hence the efficiency are substantially enhanced as compared to known processes. To solve this object, the process according to the invention essentially consists in that the volume ratio of molten slag to iron bath is chosen to be larger than 0.5 to 1 and, preferably, 0.8:1 to 1.5:1 and that the slags are supplemented with SiO.sub.2 carriers such as, e.g., foundry sands, metallurgical sands and/or fine ores so as to adjust a slag basicity (CaO/SiO.sub.2) of between 1.0 and 1.8 and, preferably, 1.2 and 1.8 at an Al.sub.2 o.sub.3 content of between 10 and 25% by weight, based on the slag, wherein hot blast is top-blown and coal, optionally along with an inert gas and, in particular, nitrogen and, furthermore, optionally oxygen or hot air is blown through the iron bath. By using an extremely high specific amount of slag, which is substantially elevated as compared to known processes, the slag is able to assume the function of a heat transfer medium from the gas space to the slag iron melt mixture. The thermal capacity of the slag, which is 1.5 to 2 time higher than that of iron, allows for a high heat transfer, thereby avoiding the blowing through of coals and hence too small an exchange surface between the reductant carrier iron bath and the oxide carrier slag due to an accordingly high iron bath portion. By adding SiO.sub.2 carriers to the slags for adjusting a defined slag basicity of from 1 to 1.8 and, in a particularly preferred manner, between 1.3 and 1.6 as provided by the invention, it is feasible to directly produce environmentally safe slags, wherein the direct usability of such environmentally safe slags may be enhanced even further by adjusting the Al.sub.2 O.sub.3 content to values of between 10 and 25% by weight, based on the slag.
By top-blowing hot blast or hot air, the heat transfer is additionally increased, the top-blowing of hot blast in combination with the blowing in of coal and N2 and, separately, oxygen into the iron bath enabling the intensive thorough mixing of the slag with the iron bath and hence an elevated heat transmission in the slag and iron bath suspension formed by thorough mixing.
In order to definitely prevent the risk of blow-throughs, it is advantageously proceeded in a manner that the melt bath height corresponds to at least 20 times and, preferably, 30 to 60 times the diameter of the submerged tuyeres. This is, at the same time, beneficial to appropriate fluidization as well as the formation of a slag and iron bath suspension, thus enhancing heat transmission in the liquid phase.
Heat transfer may be further improved by the impact pulse of the hot blast jet, such an impact pulse further improving the intensive thorough mixing of the slag bath with the iron bath. Advantageously, it is proceeded in a manner that the speed of the hot blast is chosen between 0.4 and 0.8 Mach.
An environmentally safe slag product capable of being utilized in a particular suitable manner may be produced within the scope of the process according to the invention in that the slags are granulated forming a glass content of more than 90% and, preferably, more than 93%.
In addition to the selection of the optimum hot blast speed, heat transfer may still be enhanced by increasing the specific bath surface. Such an increase in the specific bath surface may, for instance, be effected by droplets flying into the gas space of the converter using high-pressure submerged tuyeres, thereby being able to obtain an increase in the bath surface by a factor 20 as compared to a calm slag melt. And his is exactly what is reached by dimensioning the height of the melt bath relative to the diameter of the submerged tuyeres.
Unlike usual blast furnace processes and conventional steel or pig iron production processes, which aim for specific slag amounts as small as possible, the process according to the invention in a particularly advantageous manner renders feasible the conversion of a number of hitherto hardly usable iron carriers to pig iron in an energetically particularly favourable and hence economic manner. To a high degree, this applies to fine ores, which have been difficult to work up in an economic manner so far. Also dusts from steel production, which have relatively high heavy metal contents in addition to high iron contents, may be readily utilized within the scope of the process according to the invention. Naturally, this applies also to mill scales, the conventional utilization of which partially involves greater problems due to adhering oils, which do not constitute any difficulties within the scope of the process according to the invention. (Addition through central tube of hot blast lance).
The process according to the invention in the first place is of particularly great interest to electric steel works, the latter using no pig iron technology and, in particular, no blast furnace technology. Electric steel works, as a rule, must buy pig iron at relatively high prices if higher-quality products are to be produced. This holds, in particular, if the contents of copper and tin dragged into an electric furnace through scrap steel are to be reduced by dilution. Besides the favourable utilization and disposal of metallurgical residual substances such as electric furnace slag and dusts, scales as well as optionally foundry sand, also aluminium (oxide) containing grinding dusts as well as dried red muds occurring in the recovery of bauxite by the Bayer process and other sources of residual substances difficult to dispose of may be used within the scope of the process according to the invention in a particularly advantageous manner.
Advantageously, the process according to the invention is carried out in a manner that coal in an amount of 60 to 350 kg/t slag along with 6 to 9 Nm.sup.3 /t slag nitrogen as a carrier gas as well as 25 to 100 Nm.sup.3 /t slag oxygen, optionally together with hydrocarbons for protecting the tuyeres, are passed through the bath, thus rendering the economoy particularly attractive.
In order to obtain as intensive a thorough mixing of the slag and iron bath as possible, for instance, in the form of a slag and iron bath suspension, the process according to the invention advantageously is realized in a manner that the gases are fed under a pressure of 5 to 10 bars with N.sub.2 being used at a higher pressure, in particular 7 to 10 bars, than O.sub.2 and hydrocarbons such as, e.g., natural gas. A particularly high energy yield using hot blast is feasible if hot blast is top-blown through lances under a pressure of 0.8 to 1.2 bars in amounts of 400 to 1200 Nm.sup.3 /t slag.
Advantageously, the process is carried out in a manner that the converter offgas temperature is controlled at 1600 to 1800.degree. C and, preferably, 1650 to 1750.degree. C.
The process according to the invention allows for the production of environmentally compatible slags and, in particular, those having CaO to SiO.sub.2 ratios of from 1.3 to 1.6 and Al.sub.2 O.sub.3 contents of from 10 to 20% by weight at glass contents greater than 93%, which may be used in the form of mixed cements or as sulphate slag cements free of clinker. The pig iron to be produced by the process according to the invention in terms of specification corresponds to conventional blast furnace pig iron at tendentially slightly lower silicon contents. Advantageously, fine ores in amounts ranging from 200 to 1500 kg/t steel slag are charged, thereby obtaining appropriate Cu and Sn dilutions at accordingly limited offgas volumes and enabling the economic processing of fine ores.
When carrying out the process according to the invention some parameters are of particular relevance with a view to the processability and handling of the slags. Thus, it applies, in particular, that, if slags usually having basicities of about 3 in the case of steel slags, are introduced into a converter containing a metal bath, the target value of 1 to 1.8 and, preferably, about 1.5 is to be adjusted by introducing acidic SiO.sub.2 carriers. Such an alteration of the slag basicity naturally must be controlled by the lining of the converter without drastically reducing its service life while simultaneously taking into account that the slag viscosity may be substantially changed by lowering the slag basicity. Due to the decreasing basicity, the slag will be molten at lower temperatures, which in turn affects the handling of the slags such that it may appear particularly advantageous to lower the basicity of the slag to the desired extent either in steps or already prior to its introduction into the converter. While the blowing in of fine ores might thus principally be effected also via bottom tuyeres and through the metal bath, it is more advantageous--bearing in mind the above considerations--to melt open acidic components such as, for instance, fine ores, metallurgical sands or foundry sands already previously and mix them with the steel slag in a separate ladle. According to a preferred further development of the process according to the invention, the latter, therefore, is realized in a manner that the slag basicity of the steel slag is lowered in a ladle preceding the converter containing the metal bath.
In order to be able to effect the mixing of acidic additives and, in particular, SiO.sub.2 carriers with the the steel slags in a particularly efficient manner, it is advantageous to largely preheat or even melt those products. Further heating takes place during mixing due to the neutralization reaction, anyway, yet such heating may be used, in particular, in order to do without an additional heating and, in particular, an electric heating in the preceding ladle. Advantageously, the process according to the invention, therefore, is carried out such that the SiO.sub.2 carriers required for lowering the basicity such as, e.g., foundry sands, metallurgical sands and/or fine ores are heated in a melting cyclone by the converter offgases while simultaneously purifying the same. The use of a melting cyclone not only has the advantage that the sensible heat of the converter of fgases can be efficiently used, but, at the same time, offers the advantage that the converter offgases can be subjected to effective purification, the purified offgases leaving the melting cyclone thus being able to be conducted directly through heat exchangers or regenerative heat exchangers. As in correspondence with a further preferred embodiment, the Co content, and optionally the H.sub.2 content, of the converter off-gases may additionally be burnt in such a melting cyclone, thus also using the chemical heat completely.
In order to ensure that the SiO.sub.2 carriers and, in particular, foundry sands or metallurgical sands introduced are melted at the temperatures to be attained in the melting cyclone, iron oxide carriers such as, for instance, fine ores may advantageously be added. On the whole, also fine ores having the appropriate chemical compositions may be charged as sole SiO.sub.2 carriers. Advantageously, the process according to the invention is carried out in a manner that the melting cyclone containing the SiO.sub.2 carriers is supplemented with fine ores or Fe.sub.x O.sub.y carriers so as to form fayalitic slags and the formed melt is mixed with the slag melt prior to charging into the converter. Unlike quartz sands, which have melting temperatures of about 2000.degree. C., fayalitic slags have melting temperatures of but 1200.degree. to 1250.degree. C., whereby it is ensured that a melt will be obtained with the converter offgas temperature. Such a melt, which efficiently bonds in the dusts of the converter offgases at the same time, subsequently may be rapidly mixed with the steel slag melt in a ladle such that the residence time within the ladle and hence the load on the ladle may be kept low.
Advantageously, also Al.sub.2 O.sub.3 carriers such as bauxite and/or metallurgical dusts may be charged into a melting cyclone of that type. In principle, Al.sub.2 O.sub.3 carriers may, however, be blown directly into the consecutively arranged converter even without considerably stressing the lining, provided they are present in an appropriate form capable of being conveyed pneumatically.
If, within the scope of charging steel slags, precious steel slags are charged, an accordingly high chromium content will have to be taken into account, as a rule. In order to obtain slags that can be charged directly, it must be safe-guarded that such a chromium content remains within the metal bath and, therefore, dephosphorization of the metal bath, preferably under reducing conditions, must be effected. To this end, it is advantageously proceeded in a manner that the metal bath of the converter is drawn off and dephosphorized separately under reducing conditions by aid of Cao, CaC.sub.2, metallic magnesium, metallic calcium and/or CaF.sub.2.
In carrying out the process according to the invention, the following technical parameters are advantageously observed.
Technical Parameters
Submerged Tuyeres (Control Range)
Coal 60-350 kg/t.sub.slag Oxygen 25-100 Nm.sup.3 /t.sub.slag Nitrogen (carrier gas for coal) 6-9 Nm.sup.3 /t.sub.slag Natural gas (tuyere protection gas 7-10 Nm.sup.3 /t.sub.slag for oxygen feed) Oxygen and natural gas 5-8 bars Nitrogen 7-10 bars
Hot Blast Lance (Control Range)
 Pre-pressure hot blast lance 0.8-1.2 bars Hot blast 400-1200 Nm.sup.3 /t.sub.slag
Converter Offgas
 Temperature 1650-1750.degree. C. Composition 50-55% N.sub.2 7-20% CO 18-25% CO.sub.2 0.5-5% H.sub.2 7-12% H.sub.2 O Quantity 400-1200 Nm.sup.3 /t.sub.slag
The following materials were used as charging substances for the process according to the invention within the scope of an exemplary embodiment
Charging Substances
 Coal (e.g. Zentralkokerei Saar, DIN 23003) LOI 19 (%) Ashes 8 (%) Coal - Ash Analysis (%) SiO.sub.2 52 Al.sub.2 O.sub.3 25 TiO.sub.2 1 Fe.sub.2 O.sub.3 10 CaO 8 OBM Slag (NMH) (%) P.sub.2 O.sub.5 1.5 CaO 48 MnO 3 Al.sub.2 O.sub.3 4 Fe (FeO) 15 (22) Fe, met 8 Cr.sub.2 O.sub.3 1 MgO 3 SiO.sub.2 15.4 TiO.sub.2 1 Iron Ore (%) LOI 3.2 SiO.sub.2 2.5 Al.sub.2 O.sub.3 1.5 P.sub.2 O.sub.5 0.1 CaO 0.1 Fe.sub.2 O.sub.3 92 MgO 0.03 SO.sub.3 0.03 Bauxite (%) Al.sub.2 O.sub.3 95 Sand (%) SiO.sub.2 98
Steel slags produced in bottom-blowing converters, as a rule, have lower portions of FeO and metallized iron than, for instance, LD or electric furnace slags. The economy of the process according to the invention could be enhanced by adding SiO.sub.2 and Al.sub.2 O.sub.3 additives such as foundry sands, grinding dusts from the automobile industry or from engine manufacturing or the like and by adding iron carriers such as fine ores, converter dusts, mill scales as well as by using optionally dioxin-loaded active coke through the submerged tuyeres. The converter was charged with 580 kg steel works slag, 280 kg iron ore, 60 kg bauxite and 80 kg sand, the converter having been operated with 185 kg coal, 48 Nm.sup.3 oxygen and 670 Nm.sup.3 hot blast. A yield of 313 kg pig iron and 615 kg of an environmentally safe slag could be obtained.
The tapping temperatures of the produced slag and pig iron were chosen at 1500.degree. C. The initial temperature of the slag was 1400.degree. C.
Fine ore was blown in by means of oxygen submerged tuyeres, thus causing the blown-in iron ore to directly get into contact with the reducing iron bath and hence being metallized. As in contrast to fine ore charging from top using blowing lances, such an iron ore charging through bottom tuyeres has the advantage that the formation of dust is substantially reduced, thus accordingly reducing the respective delivery or emission from the converter.
By being blown into the bath as provided by the invention, fine ore can be metallized at once without having previously dissolved in the slag. In that manner, the service life of the refractory lining is substantially enhanced.
The blowing in of oxygen and carbon or other additives into the bath through separate submerged tuyere systems has proved to be unproblematic, only bivalent iron in the iron ore having not been oxidized if at all. Bauxite, sands and fine ores may be blown in or top-blown together with oxygen. Coal and, in particular, mixtures of equivalent portions of bright-burning coal and anthrazite may be blown through the bath by aid of an inert gas such as, e.g., nitrogen.
Within the scope of the process according to the invention, up to 15 to 30 kg iron ore having grain sizes of smaller than 5 mm/Nm.sup.3 oxygen could be conveyed. On the whole, both the required amount of ore and the required amount of bauxite and sand as well as additives may be introduced into the converter by aid of oxygen through the submerged tuyeres within the scope of the process according to the invention. If charging materials difficult to convey pneumatically such as, for instance, additives or ores are to be introduced additionally, this may be effected directly into the converter mouth through the central tube of the hot air lance or via a chute.